Operations, Administration and Maintenance (OAM) is a term used to describe processes, activities, tools, standards and so on that are involved with operating, administering, managing and maintaining a communication network. OAM requires fault management and performance monitoring, connectivity fault management and link layer discovery.
CFM is a protocol of OAM that provides Connectivity Fault Management (CFM). The CFM protocol uses Maintenance Domains (MD) for monitoring levels of service providers, core networks or system operators. Each level has Maintenance Associations (MA) dedicated to monitoring specific provider/provider or provider/customer service. Each MA depends on a set of Maintenance Points (MPs) for monitoring. An MA is established to verify the integrity of a single service instance. A Maintenance Association Edge Point (MEP) is an actively managed CFM entity which provides the extent of an MA and is associated with a specific port of a service instance. It can generate and receive CFM Protocol Data Units (PDUs) and track any responses. It is an end point of a single MA, and is an endpoint for each of the other MEPs in the same MA.
CFM PDUs are transmitted by a MEP in order to monitor the service to which the transmitting MEP belongs. A problem arises in Equal-Cost Multiple Paths routing (ECMP) networks where CFMs cannot be guaranteed to take the same path as the data.
ECMP routing is a forwarding mechanism for routing packets along multiple paths of equal cost. An aim of ECMP is to equalise distributed link load sharing. Referring to FIG. 1, there is illustrated schematically a very simple network architecture in which data sent between two endpoints 1, 2 can be sent via intermediate nodes 3 or 4 at equal cost.
Some data packets are sent via intermediate node 3 and others are sent via intermediate node 4. In this way the load on the network is balanced.
Shortest Path Bridging (SPB) enables the use of link state protocols (IS-IS) for constructing active topologies within a Bridged Network. For more information, see IEEE Std 802.1aq-2012, Shortest Path Bridging. Recent standardization work within IEEE802.1 enhances SPB by enabling ECMP support on SPBM services that use the same VID identifier, as described in P802.1Qbp/D1.0 Equal Cost Multiple Path (ECMP). P802.1Qbp discusses the services supporting ECMP connectivity and, in particular, defines two types of ECMP connectivity. One is associated with point-to-point (PtP) ECMP services provided by ECMP devices that support flow filtering. The other is a generic Virtual LAN (VLAN) service associated with a specific VLAN Identifier (VID) that is mapped to ECMP operation (SPBM VLAN MA in clause 27.18.1 in P802.1Qbp/D1.0 Equal Cost Multiple Path).
ECMP connectivity paths may use the same Bridging VLAN Identifier (B-VID) in their tags but the service connectivity provided by these paths are different than that associated with frames having the same B-VID and controlled by traditional L2 control protocols like spanning tree or SPB. A typical example of connectivity instances that use the same VID but are not members of a VLAN are Traffic Engineered Service Instances (TESIs) in Provider Backbone Bridges-Traffic Engineering (PBB-TE). ECMP connectivity is similar to that of TESIs but it has a further property that a superset of all ECMP paths identified by the same VID (and endpoints) is not a tree topology. A VLAN on the other hand is always defined in a context of a tree (see clause 7 in IEEE Std 802.1Q-2011, VLAN aware Bridges).
Shortest Path Bridging-MAC address mode (SPBM) connectivity is different to ECMP connectivity. SPBM connectivity is similar to that of PBB-TE (in that it there is no flooding, no learning, it is symmetric, and uses only explicit entries in a Filtering Database (FDB) for forwarding), which means in practice that CFM enhancements for PBB-TE (described in IEEE Std 802.1Qay-2009 PBB-TE and IEEE Std 802.1Q-2011, VLAN aware Bridges) can be used almost identically for SPBM MAs. Nevertheless, ECMP connectivity differs in that multiple paths are enabled for the same end points. The same VID and correspondingly the ECMP CFM require further changes in order to monitor the associated services. As a result ECMP MAs need to be separated from SPBM MAs, and the associated monitoring protocol tools need to be modified as their operation depends on the type of connectivity that they monitor.
ECMP Point-to-Point (PtP) path connectivity and the associated monitoring tools are described in P802.1Qbp, but P802.1Qbp does not describe ECMP multipoint monitoring in a consistent manner. In particular, the “SPBM VLAN” connectivity is associated with an overall connectivity identified by the same SPBM VID value. However, an overall SPBM-VID connectivity is meaningless for ECMP, because ECMP creates multiple independent connectivity paths between subsets of nodes that are members of the SPBM-VID. The operational status of each of the ECMP subsets is therefore independent of the operational status of the other ECMP subsets identified by the same SPBM-VID. This ECMP independency means that, when using SPBM OAM mechanisms and an overall SPBM-VID connectivity is reported as being error-free, the connectivity on ECMP subsets could be non-operational. The above connectivity characteristic of the SPBM VLAN Maintenance Association (MA) creates problems for monitoring multipoint ECMP services. In particular, since the SPBM VLAN Continuity Check protocol attempts to monitor the overall “VLAN” service, the scope of propagation of the Continuity Check Message (CCM) PDUs is provided by the use of a broadcast address (constructed using SPBM default Backbone Service Identifier, I-SID). The result of this is that monitored connectivity is different from the connectivity associated with the monitored data traffic. In addition, the operation of Link Trace Messages (LTM) becomes quite difficult and the extent of reachability of the LTMs can be quite different to that defined by the configured ECMP related MAC address entries.
Furthermore, the placement of the “SPBM VLAN MEP” in parallel to ECMP PtP path Maintenance Association Edge Points (MEPs) breaks the operation of the ECMP path MAs (stopping every ECMP Path CFM PDU on the SPBM-VID as can be seen from FIG. 27-4 in P802.1Qbp/D1.0.)